Fix helm namespace lifecycle in tests
Summary: I think the intention of this contextmanager is to leave the helm resources for the whole lifecycle, but currently its returning but not yielding so that doesn't happen. Make it a contextmanager so that it will wait until the end of the test to cleanup.
Test Plan: k8s integration tests now pass locally again
Reviewers: catherinewu, nate, johann
Reviewed By: catherinewu
Differential Revision: https://dagster.phacility.com/D4804